There is significant demand for high power narrow linewidth laser sources, with wavelength that extends beyond the gain bandwidth of existing rare earth ions (REIs), such as, for example, ytterbium (1 micron), erbium (1.55 micron), and thulium (˜2 micron)
While Raman scattering in optical fiber is useful to obtain gain at any wavelength within the transparency window of the fiber with a suitable choice of the pump wavelength, and all-fiber Raman laser resonators have been demonstrated using Bragg reflectors, as well as other types of resonant cavities, as illustrated in FIG. 1 and of which a transmission spectrum is illustrated in FIG. 2, it has not yet been possible to achieve oscillation with linewidth narrower than ˜1 GHz. Although Raman distributed feedback (DFB) lasers have been proposed theoretically to produce single frequency generation, this has not been demonstrated to date due to severe design deficiencies and practical limitations, as described below.
In practice, a number of problems have made it difficult to achieve single frequency or narrow linewidth Raman lasing in DFB laser structures. These include:                (1) loss due to the UV exposure during grating writing,        (2) difficulty in fabricating long gratings with high uniformity,        (3) Kerr-nonlinearity induced Bragg-wavelength variation,        (4) variations due to non-uniform thermal distribution along the grating and within the fiber, causing refractive index changes, for example, and        (5) high required pump powers.        
In general, use of fiber Raman gain to generate narrowband signals at wavelengths away from REI gain bandwidths has not been achieved with high power using available pumps in conventional laser cavities designed using fiber Bragg gratings.
Even with attention to critical details, the demonstrated practical performance has resulted in (1) a large power threshold for lasing, (2), a small output power, and (3) inefficient conversion of pump power into a single frequency or suitably narrow linewidth signal.
Note, that although much of this document discusses fiber waveguides, the inventive concepts are applicable to other types of waveguides, such as planar waveguides and others, as well. Furthermore, it's possible that lasing can be achieved even without waveguides formed using mechanisms such as gain guiding or thermal lensing given the high optical intensities.